1. Field of the Invention
The present invention relates to a PN junction type solar battery and, more particularly, to a solar battery consisting mainly of PN junction semiconductor elements and a method of producing the same.
2. Description of the Prior Art
A conventional solar battery of this type heretofore well known will be hereinafter described with reference to FIG. 1.
As shown, a conventional solar battery has an N-type silicon substrate 1, a P-type diffused layer 2 formed on the surface of the silicon substrate 1, and terminal electrodes 3 and 4 provided on the rear side of the N-type board 1 and on a part of the P-type diffused layer 2, respectively, in ohmic contact therewith so that the photoelectromotive force may be obtained.
In the case of such a conventional solar battery, however, the PN junction 5 is formed at a deep position relative to the plane on which light rays A are incident the surface of the P-type layer 2 and therefore electron-hole pairs produced by the light rays A are mostly recombined and disappear before reaching the above-mentioned junction 5 and thereby the photoelectric conversion efficiency is extremely deteriorated. Especially, almost all the electron-hole pairs produced in the vicinity of the surface of the P-type layer are recombined before reaching the above junction 5, and therefore the photoelectric conversion efficiency for the short wavelength components of the incident rays of light A is inevitably very low.
In other words, the conventional solar battery of the type such as shown in FIG. 2 carries out the conversion of only the long wavelength components of the light rays having the spectral distribution curve "a" shown in FIG. 7, as shown by the dot-dash curve "b".
In order to improve the conversion efficiency of the light rays in the short wavelength region, meanwhile, an improved version of the solar battery shown in FIG. 1 is known in which the P-type layer 2 is formed by special film-making techniques such as gaseous- or liquid-phase epitaxy, evaporation and ion injection.
In the case of the solar battery which has the P-N junction 5 formed by the above-mentioned film-making techniques, however, the electron-hole pairs produced at the positions deeper than the junction 5 by long wavelength rays of light are recombined before reaching the junction 5 and therefore the conversion efficiency is lowered in the long wavelength region; accordingly, though the photoelectric conversion characteristic is very much improved in the short wavelength region as shown in FIG. 7 by the broken curve "c", it is not improved as a whole.
Besides the above-mentioned solar battery as shown in FIG. 1, what is called the vertical multi-junction solar cell has recently been developed. As shown in FIG. 2, the solar battery of this type has a number of P-type layers 11P and N-type layers 11N formed alternately with each other, the P-N junctions 12 thereof being vertical to the plane on which the rays of light A are incident; accordingly, if the thicknesses of the above P-type layer 11P and N-type layer 11N are made less than twice the diffusion length of the respective minority carriers, almost all the electron-hole pairs produced at positions deep in the battery as well as in the vicinity of the light-receiving plane will be transported to the P-N junction 12 thereby to remarkably improve the photoelectric conversion efficiency.
In such a solar battery as shown in FIG. 2, the thickness of each layer inevitably becomes small for the above-mentioned structural reason and is desirably less than twice the diffusion length of the minority carriers therein; therefore a solar battery of this type requires in its production a very difficult-to-perform technique for controlling the layer thickness and providing terminals 13 and 14; for this reason, it cannot be mass-produced as yet and therefore is expensive.
In view of the above-mentioned drawbacks, the present invention contemplates the elimination of the above-mentioned disadvantages of the prior art and to provide a novel solar battery.